Severely sterically hindered tertiary amino compounds

ABSTRACT

A novel class of severely sterically hindered tertiary amino compounds having a pK a  value at 20° C. greater than 8.6, preferably ranging from 9.3 to about 10.6, and a cumulative -E s  value of at least 1.9 are found to be useful in selective removal of H 2  S from a normally gaseous mixture containing H 2  S and CO 2 .

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to severely sterically hindered tertiaryamino compounds and their preparation. These compounds are useful inselective removal of hydrogen sulfide from gaseous streams containinghydrogen sulfide and carbon dioxide.

2. Description of Related Patents

It is well known in the art to treat gases and liquids, such as mixturescontaining acidic gases including CO₂, H₂ S, CS₂, HCN, COS and oxygenand sulfur derivatives of C₁ to C₄ hydrocarbons with amine solutions toremove these acidic gases. The amine usually contacts the acidic gasesand the liquids as an aqueous solution containing the amine in anabsorber tower with the aqueous amine solution contacting the acidicfluid countercurrently.

The treatment of acid gas mixtures containing, inter alia, CO₂ and H₂ Swith amine solutions typically results in the simultaneous removal ofsubstantial amounts of both the CO₂ and H₂ S. For example, in one suchprocess generally referred to as the "aqueous amine process," relativelyconcentrated amine solutions are employed. A recent improvement on thisprocess involves the use of sterically hindered amines as described inU.S. Pat. No. 4,112,052, to obtain nearly complete removal of acid gasessuch as CO₂ and H₂ S. This type of process may be used where the partialpressures of the CO₂ and related gases are low. Another process oftenused for specialized applications where the partial pressure of CO₂ isextremely high and/or where many acid gases are present, e.g., H₂ S,COS, CH₃ SH and CS₂ involves the use of an amine in combination with aphysical absorbent, generally referred to as the "nonaqueous solventprocess." An improvement on this process involves the use of stericallyhindered amines and organic solvents as the physical absorbent such asdescribed in U.S. Pat. No. 4,112,051.

It is often desirable, however, to treat acid gas mixtures containingboth CO₂ and H₂ S so as to remove the H₂ S selectively from the mixture,thereby minimizing removal of the CO₂. Selective removal of H₂ S resultsin a relatively high H₂ S/CO₂ ratio in the separated acid gas whichsimplifies the conversion of H₂ S to elemental sulfur using the Clausprocess.

The typical reactions of aqueous secondary and tertiary amines with CO₂and H₂ S can be represented as follows:

    H.sub.2 S+R.sub.3 N⃡R.sub.3 NH.sup.+ +SH.sup.- ( 1)

    H.sub.2 S+R.sub.2 NH⃡R.sub.2 NH.sub.2.sup.+ +SH.sup.-( 2)

    CO.sub.2 +R.sub.3 N+H.sub.2 O⃡R.sub.3 NH.sup.+ +HCO.sub.3.sup.-( 3)

    CO.sub.2 +2R.sub.2 NH⃡R.sub.2 NH.sub.2.sup.+ +R.sub.2 NCOO.sup.-( 4)

wherein R is an organic radical which may be the same or different andmay be substituted with a hydroxy group. The above reactions arereversible, and the partial pressures of both CO₂ and H₂ S are thusimportant in determining the degree to which the above reactions occur.

While selective H₂ S removal is applicable to a number of gas treatingoperations including treatment of hydrocarbon gases from shalepyrolysis, refinery gas and natural gas having a low H₂ S/CO₂ ratio, itis particularly desirable in the treatment of gases wherein the partialpressure of H₂ S is relatively low compared to that of CO₂ because thecapacity of an amine to absorb H₂ S from the latter type gases is verylow. Examples of gases with relatively low partial pressures of H₂ Sinclude synthetic gases made by coal gasification, sulfur plant tail gasand low-Joule fuel gases encountered in refineries where heavy residualoil is being thermally converted to lower molecular weight liquids andgases.

Although it is known that solutions of primary and secondary amines suchas monoethanolamine (MEA), diethanolamine (DEA), dipropanolamine (DPA),and hydroxyethoxyethylamine (DGA) absorb both H₂ S and CO₂ gas, theyhave not proven especially satisfactory for preferential absorption ofH₂ S to the exclusion of CO₂ because the amines undergo a facilereaction with CO₂ to form carbamates.

Diisopropanolamine (DIPA) is relatively unique among secondaryaminoalcohols in that it has been used industrially, alone or with aphysical solvent such as sulfolane, for selective removal of H₂ S fromgases containing H₂ S and CO₂, but contact times must be kept relativelyshort to take advantage of the faster reaction of H₂ S with the aminecompared to the rate of CO₂ reaction shown in Equations 2 and 4hereinabove.

In 1950, Frazier and Kohl, Ind. and Eng. Chem., 42, 2288 (1950) showedthat the tertiary amine, methyldiethanolamine (MDEA), has a high degreeof selectivity toward H₂ S absorption over CO₂. This greater selectivitywas attributed to the relatively slow chemical reaction of CO₂ withtertiary amines as compared to the rapid chemical reaction of H₂ S. Thecommercial usefulness of MDEA, however, is limited because of itsrestricted capacity for H₂ S loading and its limited ability to reducethe H₂ S content to the level at low pressures which is necessary fortreating, for example, synthetic gases made by coal gasification.

Recently, U.K. Patent Publication 2,017,524A to Shell disclosed thataqueous solutions of dialkylmonoalkanolamines, and particularlydiethylmonoethanolamine (DEAE), have higher selectivity and capacity forH₂ S removal at higher loading levels than MDEA solutions. Nevertheless,even DEAE is not very effective for the low H₂ S loading frequentlyencountered in the industry. Also, DEAE has a boiling point of 161° C.,and as such, it is characterized as being a low-boiling, relativelyhighly volatile amino alcohol. Such high volatilities under most gasscrubbing conditions result in large material losses with consequentlosses in economic advantages.

SUMMARY OF THE INVENTION

It has now been discovered that absorbent solutions of a certain classof amino compounds defined as severely sterically hindered tertiaryamino compounds have a high selectivity for H₂ S compared to CO₂. Theseamino compounds surprisingly maintain their high selectivity at high H₂S and CO₂ loadings.

The present invention relates to a specific class of severely stericallyhindered tertiary amino compounds of the general formula: ##STR1##wherein R₁ and R₂ are each independently selected from the groupconsisting of alkyl having 1 to 8 carbon atoms and hydroxyalkyl radicalshaving 2 to 8 carbon atoms and cycloalkyl and hydroxycycloalkyl radicalshaving 3 to 8 carbon atoms, R₃, R₄, R₅ and R₆ are each independentlyselected from the group consisting of hydrogen and C₁ -C₄ alkyl andhydroxyalkyl radicals, with the proviso that if the carbon atom of R₁directly attached to the nitrogen atom is secondary and the carbon atomof R₂ directly attached to the nitrogen atom is primary, at least one ofR₃ or R₄ directly bonded to the carbon which is bonded to the nitrogenis an alkyl or hydroxyalkyl radical, x and y are each positive integersindependently ranging from 2 to 4, and z is either zero or a positiveinteger ranging from 1 to 4.

Another embodiment of the invention relates to a process for preparingthe amino compounds herein which comprises reacting the correspondingseverely sterically hindered secondary amino alcohol of the formula:##STR2## wherein R₁, R₃, R₄, R₅, R₆, x, y and z are defined as describedabove with an aldehyde of the formula R₂ CHO wherein R₂ is definedabove. The reaction takes place at elevated temperatures in the presenceof hydrogen gas under hydrogenation conditions or in the presence of anorganic acid and inorganic acid such as formic acid and hydrochloricacid. Hydrogenation conditions ordinarily involve temperatures of, e.g.,about 60°-100° C. and pressures of up to, e.g., 1000 psi or more andinclude the presence of a Pd/C catalyst and an appropriate solvent forthe reaction such as, e.g., methanol, the product is typically recoveredby separation of the reaction mixture and distillation of the separatedportion.

By the term "severely sterically hindered" it is meant that the nitrogenatom of the amine is attached to one or more bulky carbon groupings.Typically, the tertiary amino group will have a degree of sterichindrance such that the cumulative ⁻ E_(s) value (Taft's sterichindrance constant) is at least 1.9 as calculated from the values givenfor primary amines in Table V from the article by D. F. DeTar, Journalof Organic Chemistry, 45, 5174(1980), the entire disclosure of which isincorporated herein by reference.

The above-defined compounds can be used in a process for the selectiveabsorption of H₂ S from a normally gaseous mixture containing H₂ S andCO₂ comprising:

(a) contacting said normally gaseous mixture with an absorbent solutioncomprising a severely sterically hindered tertiary amino compound havinga pK_(a) at 20° C. of at least 9.30 and a cumulative ⁻ E_(s) value(Taft's steric hindrance constant) of at least 1.9 under conditions suchthat H₂ S is selectively absorbed from said mixture;

(b) regenerating, at least partially, said absorbent solution containingH₂ S; and

(c) recycling the regenerated solution for the selective absorption ofH₂ S by contacting as in step (a).

Preferably, the regeneration step is carried out by heating andstripping and more preferably heating and stripping with steam.

The tertiary amino compound is preferably one or more compounds selectedfrom the group consisting of N-methyl-N-tertiarybutylaminoethoxyethanol,2-(N-isopropyl-N-methylamino)propoxyethanol, and3-aza-2,2,3-trimethyl-1,6-hexanediol.

The amino compounds herein are further characterized by their lowvolatility and high solubility in water at selective H₂ S removalconditions, and most of the compounds are also generally soluble inpolar organic solvent systems which may or may not contain water. Theterm "absorbent solution" as used herein includes but is not limited tosolutions wherein the amino compound is dissolved in a solvent selectedfrom water or a physical absorbent or mixtures thereof. Solvents whichare physical absorbents (as opposed to the amino compounds which arechemical absorbents) are described, for example, in U.S. Pat. No.4,112,051, the entire disclosure of which is incorporated herein byreference, and include, e.g., aliphatic acid amides, N-alkylatedpyrrolidones, sulfones, sulfoxides, glycols and the mono- and diethersthereof. The preferred physical absorbents herein are sulfones, and mostparticularly, sulfolane.

The absorbent solution ordinarily has a concentration of amino compoundof about 0.1 to 6 moles per liter of the total solution, and preferably1 to 4 moles per liter, depending primarily on the specific aminocompound employed and the solvent system utilized. If the solvent systemis a mixture of water and a physical absorbent, the typical effectiveamount of the physical absorbent employed may vary from 0.1 to 5 molesper liter of total solution, and preferably from 0.5 to 3 moles perliter, depending mainly on the type of amino compound being utilized.The dependence of the concentration of amino compound on the particularcompound employed is significant because increasing the concentration ofamino compound may reduce the basicity of the absorbent solution,thereby adversely affecting its selectivity for H₂ S removal,particularly if the amino compound has a specific aqueous solubilitylimit which will determine maximum concentration levels within the rangegiven above. It is important, therefore, that the proper concentrationlevel appropriate for each particular amino compound be maintained toinsure satisfactory results.

The solution of this invention may include a variety of additivestypically employed in selective gas removal processes, e.g., antifoamingagents, antioxidants, corrosion inhibitors, and the like. The amount ofthese additives will typically be in the range that they are effective,i.e., an effective amount.

Also, the amino compounds described herein may be admixed with otheramino compounds as a blend, preferably with methyldiethanolamine. Theratio of the respective amino compounds may vary widely, for example,from 1 to 99 weight percent of the amino compounds described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic flow sheet illustrating anabsorption-regeneration unit for selective removal of H₂ S from gaseousstreams containing H₂ S and CO₂.

FIG. 2 is a diagrammatic flow sheet illustrating an experimental spargedabsorber unit for use in rapid determination of the selectivity of theamino compound for selective removal of H₂ S from gaseous streamscontaining H₂ S and CO₂.

FIG. 3 graphically illustrates the selectivity for H₂ S plotted againstthe H₂ S and CO₂ loading for three (3) molar solutions ofN-methyl-N-tertiarybutylaminoethoxyethanol (MTBEE) and2-(N-isopropyl-N-methylamino)propoxyethanol (2-IMPE) as compared tomethyldiethanolamine (MDEA).

FIG. 4 graphically illustrates the selectivity for H₂ S plotted againstthe H₂ S and CO₂ loading for a one (1) molar solution ofN-methyl-N-tertiarybutylaminoethoxyethanol (MTBEE) as compared against a1.6 molar solution of methyldiethanolamine (MDEA).

FIG. 5 graphically illustrates the selectivity for H₂ S plotted againstthe H₂ S and CO₂ loading for a three (3) molar solution ofN-methyl-N-tertiarybutylaminoethoxyethanol (MTBEE) as compared toN-tertiarybutyldiethanolamine (TBDEA).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The compounds of the invention are preferably characterized by thefollowing formula: ##STR3## wherein R is a tertiary butyl, tertiary amylor hydroxy tertiary butyl radical, x and y are each 2-4 and z is 0 or 1.Most preferably, the amino compound herein isN-methyl-N-tertiarybutylaminoethoxyethanol.

The most preferred amino compounds herein are of the formula: ##STR4##

The amino compounds used in the process of the present invention have apK_(a) value at 20° C. greater than 8.6, preferably greater than about9.3 and more preferably the pK_(a) value of the amino compound willrange between about 9.3 and about 10.6. If the pK_(a) is less than 8.6the reaction with H₂ S is decreased, whereas if the pK_(a) of the aminocompound is much greater than about 10.6 an excessive amount of steam isrequired to regenerate the solution. Also, to insure operationalefficiency with minimal losses of the amino compound, the amino compoundshould have a relatively low volatility. For example, the boiling pointof the amine (at 760 mm) is typically greater than about 180° C.,preferably greater than 200° C., and more preferably greater than 225°C. Diethylaminoethanol is therefore unsuitable because it has a boilingpoint of 161° C.

Three characteristics which are of ultimate importance in determiningthe effectiveness of the amino compounds herein for H₂ S removal are"selectivity", "loading" and "capacity". The term "selectivity" as usedthroughout the specification is defined as the following mole ratiofraction: ##EQU1## The higher this fraction, the greater the selectivityof the absorbent solution for the H₂ S in the gas mixture.

By the term "loading" is meant the concentration of the H₂ S and CO₂gases physically dissolved and chemically combined in the absorbentsolution as expressed in moles of gas per moles of the amine. The bestamino compounds are those which exhibit good selectivity up to arelatively high loading level. The amino compounds used in the practiceof the present invention typically have a "selectivity" of notsubstantially less than 10 at a "loading" of 0.1 moles, preferably, a"selectivity" of not substantially less than 10 at a loading of 0.2 ormore moles of H₂ S and CO₂ per mole of the amino compound.

"Capacity" is defined as the moles of H₂ S loaded in the absorbentsolution at the end of the absorption step minus the moles of H₂ Sloaded in the absorbent solution at the end of the desorption step. Highcapacity enables one to reduce the amount of amine solution to becirculated and use less heat or steam during regeneration.

The acid gas mixture herein necessarily includes H₂ S, and mayoptionally include other gases such as CO₂, N₂, CH₄, H₂, CO, H₂ O, COS,HCN, C₂ H₄, NH₃, and the like. Often such gas mixtures are found incombustion gases, refinery gases, town gas, natural gas, syn gas, watergas, propane, propylene, heavy hydrocarbon gases, etc. The absorbentsolution herein is particularly effective when the gaseous mixture is agas, obtained, for example, from shale oil retort gas, coal orgasification of heavy oil with air/stream or oxygen/steam, thermalconversion of heavy residual oil to lower molecular weight liquids andgases, or in sulfur plant tail gas clean-up operations.

The absorption step of this invention generally involves contacting thenormally gaseous stream with the absorbent solution in any suitablecontacting vessel. In such processes, the normally gaseous mixturecontaining H₂ S and CO₂ from which the H₂ S is to be selectively removedmay be brought into intimate contact with the absorbent solution usingconventional means, such as a tower or vessel packed with, for example,rings or with sieve plates, or a bubble reactor.

In a typical mode of practicing the invention, the absorption step isconducted by feeding the gaseous mixture into the lower portion of theabsorption tower while fresh absorbent solution is fed into the upperregion of the tower. The gaseous mixture, freed largely from the H₂ S,emerges from the upper portion of the tower, and the loaded absorbentsolution, which contains the selectively absorbed H₂ S, leaves the towernear or at its bottom. Preferably, the inlet temperature of theabsorbent solution during the absorption step is in the range of fromabout 20° to about 100° C., and more preferably from 40° to about 60° C.Pressures may vary widely; acceptable pressures are between 5 and 2000psia, preferably 20 to 1500 psia, and most preferably 25 to 1000 psia inthe absorber. The contacting takes place under conditions such that theH₂ S is selectively absorbed by the solution. The absorption conditionsand apparatus are designed so as to minimize the residence time of theliquid in the absorber to reduce CO₂ pickup while at the same timemaintaining sufficient residence time of gas mixture with liquid toabsorb a maximum amount of the H₂ S gas. The amount of liquid requiredto be circulated to obtain a given degree of H₂ S removal will depend onthe chemical structure and basicity of the amino compound and on thepartial pressure of H₂ S in the feed gas. Gas mixtures with low partialpressures such as those encountered in thermal conversion processes willrequire more liquid under the same absorption conditions than gases withhigher partial pressures such as shale oil retort gases.

A typical procedure for the selective H₂ S removal phase of the processcomprises selectively absorbing H₂ S via countercurrent contact of thegaseous mixture containing H₂ S and CO₂ with the solution of the aminocompound in a column containing a plurality of trays at a lowtemperature, e.g., below 45° C., and at a gas velocity of at least about0.3 ft/sec (based on "active" or aerated tray surface), depending on theoperating pressure of gas, said tray column having fewer than 20contacting trays, with, e.g., 4-16 trays being typically employed.

After contacting the normally gaseous mixture with the absorbentsolution, which becomes saturated or partially saturated with H₂ S, thesolution may be at least partially regenerated so that it may berecycled back to the absorber. As with absorption, the regeneration maytake place in a single liquid phase. Regeneration or desorption of theabsorbent solution may be accomplished by conventional means such aspressure reduction of the solution or increase of temperature to a pointat which the absorbed H₂ S flashes off, or by passing the solution intoa vessel of similar construction to that used in the absorption step, atthe upper portion of the vessel, and passing an inert gas such as air ornitrogen or preferably steam upwardly through the vessel. Thetemperature of the solution during the regeneration step should be inthe range from about 50 to about 170° C., and preferably from about 80°to 120° C., and the pressure of the solution on regeneration shouldrange from about 0.5 to about 100 psia, preferably 1 to about 50 psia.The absorbent solution, after being cleansed of at least a portion ofthe H₂ S gas, may be recycled back to the absorbing vessel. Makeupabsorbent may be added as needed.

In the preferred regeneration technique, the H₂ S-rich solution is sentto the regenerator wherein the absorbed components are stripped by thesteam which is generated by re-boiling the solution. Pressure in theflash drum and stripper is usually 1 to about 50 psia, preferably 15 toabout 30 psia, and the temperature is typically in the range from about50° to 170° C., preferably about 80° C. to 120° C. Stripper and flashtemperatures will, of course, depend on stripper pressure, thus at about15 to 30 psia stripper pressures, the temperature will be about 80° toabout 120° C. during desorption. Heating of the solution to beregenerated may very suitably be effected by means of indirect heatingwith low-pressure steam. It is also possible, however, to use directinjection of steam.

In one embodiment for practicing the entire process herein, asillustrated in FIG. 1, the gas mixture to be purified is introducedthrough line 1 into the lower portion of a gas-liquid countercurrentcontacting column 2, said contacting column having a lower section 3 andan upper section 4. The upper and lower sections may be segregated byone or a plurality of packed beds as desired. The absorbent solution asdescribed above is introduced into the upper portion of the columnthrough a pipe 5. The solution flowing to the bottom of the columnencounters the gas flowing countercurrently and dissolves the H₂ Spreferentially. The gas freed from most of the H₂ S exits through a pipe6, for final use. The solution, containing mainly H₂ S and some CO₂,flows toward the bottom portion of the column, from which it isdischarged through pipe 7. The solution is then pumped via optional pump8 through an optional heat exchanger and cooler 9 disposed in pipe 7,which allows the hot solution from the regenerator 12 to exchange heatwith the cooler solution from the absorber column 2 for energyconservation. The solution is entered via pipe 7 to a flash drum 10equipped with a line (not shown) which vents to line 13 and thenintroduced by pipe 11 into the upper portion of the regenerator 12,which is equipped with several plates and effects the desorption of theH₂ S and CO₂ gases carried along in the solution. This acid gas ispassed through a pipe 13 into a condenser 14 wherein cooling andcondensation of water and amine solution from the gas occur.

The gas then enters a separator 15 where further condensation iseffected. The condensed solution is returned through pipe 16 to theupper portion of the regenerator 12. The gas remaining from thecondensation, which contains H₂ S and some CO₂, is removed through pipe17 for final disposal (e.g., to a vent or incinerator or to an apparatuswhich converts the H₂ S to sulfur, such as a Claus unit or a Stretfordconversion unit (not shown)).

The solution is liberated from most of the gas which it contains whileflowing downward through the regenerator 12 and exits through pipe 18 atthe bottom of the regenerator for transfer to a reboiler 19. Reboiler19, equipped with an external source of heat (e.g., steam is injectedthrough pipe 20 and the condensate exits through a second pipe (notshown)), vaporizes a portion of this solution (mainly water) to drivefurther H₂ S therefrom. The H₂ S and steam driven off are returned viapipe 21 to the lower section of the regenerator 12 and exited throughpipe 13 for entry into the condensation stages of gas treatment. Thesolution remaining in the reboiler 19 is drawn through pipe 22, cooledin heat exchanger 9, and introduced via the action of pump 23 (optionalif pressure is sufficiently high) through pipe 5 into the absorbercolumn 2.

The amino compounds herein are found to be superior to those used in thepast, particularly to MDEA and DEAE, in terms of both selectivity andcapacity for maintaining selectivity over a broad loading range.Typically, a gaseous stream to be treated having a 1:10 mole ratio of H₂S:CO₂ from an apparatus for thermal conversion of heavy residual oil, ora Lurgi coal gas having a mole ratio of H₂ S:CO₂ of less than 1:10 willyield an acid gas having a mole ratio of H₂ S:CO₂ of about 1:1 aftertreatment by the process of the present invention. The process hereinmay be used in conjunction with another H₂ S selective removal process;however, it is preferred to carry out the process of this invention byitself, since the amino compounds are extremely effective by themselvesin preferential absorption of H₂ S.

The invention is illustrated further by the following examples, which,however, are not to be taken as limiting in any respect. All parts andpercentages, unless expressly stated to be otherwise, are by weight.

EXAMPLE 1 Preparation of N-methyl-N-tertiarybutylaminoethoxyethanol

A total of 145 g of tertiarybutylaminoethoxyethanol, 108 g of 37%aqueous formaldehyde, 10 g of 10% Pd/C, and 1 l of methanol were chargedto an autoclave, pressured up to 1000 psi with H₂ and heated at 80° C.for 8 hours. Filtration and distillation of the reaction mixture yielded128 g of N-methyl-N-tertiarybutylaminoethoxyethanol with b.p. of 128° C.at 23 mm.

EXAMPLE 2 Preparation of 2-(N-isopropyl-N-methylamino)propoxyethanol

A total of 155 g of 2-(isopropylamino) propoxyethanol, 122 g of 37%aqueous formaldehyde, 5 g of 10% Pd/C and 1 l of methanol were chargedto an autoclave, pressured to 1000 psi with H₂ and heated at 80° C. for3 hours. Filtration and distillation of the reaction mixture yielded 159g of 2-(N-isopropyl-N-methylamino)propoxyethanol with b.p. of 125° C. at20 mm.

EXAMPLE 3 Preparation of 3-aza-2,2,3-trimethyl-1,6-hexanediol

A solution of 60 g of 3-aza-2,2-dimethyl-1,6-hexanediol, 53.5 g offormic acid and 36.3 g of 37% aqueous formaldehyde was refluxed for 40hours. A total of 20 ml of concentrated hydrochloric acid was added andthe reaction mixture was distilled until the overhead temperature was110° C. Water, in an amount of 40 ml, was added to the distillationresidue and made strongly alkaline with KOH. The organic phase wasseparated and distilled to yield 39.9 g of3-aza-2,2,3-trimethyl-1,6-hexanediol with a b.p. of 103° C. at 0.35 mm,a ⁻ E_(s) value of 2.20 and a pKa of 9.35.

EXAMPLE 4 Selective H₂ S removal from a mixture containing H₂ S and CO₂

FIG. 2 illustrates the sparged absorber unit, operated on a semi-batchmode, used to evaluate the selectivity for H₂ S removal of the aminocompounds of the invention herein. A gas mixture comprised of 10% CO₂,1% H₂ S and 89% N₂, expressed in volume percent, respectively, waspassed from a gas cylinder (not shown) through line 30 to a meter 31measuring the rate at which the gas is fed to the absorber. For allexamples this rate was 3.6 liters per minute. The gas was then passedthrough line 32 to a gas chromatography column (not shown) continuouslymonitoring the composition of the inlet gas and through lines 33 and 34to a sparged absorber unit 35, which is a cylindrical glass tube 45 cmhigh and 3.1 cm in diameter charged with 100 ml of the absorbent aminesolution 36. The gas was passed through the solution at a solutiontemperature of 40° C., and 10-ml samples of the solution wereperiodically removed from the bottom of the absorber unit through lines34 and 37 to be analyzed for H₂ S and CO₂ contents. The H₂ S content inthe liquid sample was determined by titration with silver nitrate. TheCO₂ content of the liquid sample was analyzed by acidifying the samplewith an aqueous solution of 10% HCl and measuring the evolved CO₂ byweight gain on NaOH-coated asbestos.

While the solution was being periodically withdrawn from the bottom ofthe absorber unit, the gas mixture was removed from the top thereof vialine 38 to a trap 39 which served to scrub out any H₂ S in the outletgas. The resulting gas could optionally then be passed via lines 40 and41 for final disposal or via line 42 to a gas chromatography column (notshown) for periodic evaluation of the composition of the outlet gas tocheck for system leaks. For purposes of the examples, the H₂ S and CO₂contents of the inlet gas phase were measured, and the H₂ S and CO₂contents of the liquid phase were determined as described above. Thesedata were used to calculate selectivity values of the amine as definedabove, which were plotted as a function of the loading of the absorbentsolution with H₂ S and CO₂, in units of moles acid gas per moles of theamino compound.

In the example aqueous 3 M solutions ofN-methyl-N-tertiarybutylaminoethoxyethanol (MTBEE) (⁻ E_(s) =2.17,pK_(a) =10.15) and 2-(N-isopropyl-N-methylamino)propoxyethanol(2-IMPE)(⁻ E_(s) =1.93, pK_(a) =9.55) were compared with an aqueous 3 M solutionof methyldiethanolamine (MDEA) as control using the same gas mixture andconditions. From the plots of selectivity for H₂ S removal and loadingshown in FIG. 3 it can be seen that MTBEE and 2-IMPE have a higher H₂ Sselectivity than MDEA.

EXAMPLE 5

The procedure of Example 4 was repeated except that a 1 M aqueoussolution of N-methyl-N-tertiarybutylaminoethoxyethanol (MTBEE) wascompared against a 1.6 M aqueous solution of methyldiethanolamine (MDEA)as control using the same gas mixture and conditions. From the plots ofselectivity for H₂ S removal shown in FIG. 4, it can be seen that thecapacity for H₂ S of MTBEE increases with decreasing solutionconcentration, whereas the capacity for H₂ S of MDEA decreases withdecreasing concentration.

EXAMPLE 6

The procedure of Example 4 was repeated except that an aqueous 3 Msolution of N-methyl-N-tertiarybutylaminoethoxyethanol (MTBEE) wascompared with N-tertiarybutyldiethanolamine (TBDEA) (⁻ E_(s) =2.46,pK_(a) =8.2) as control using the same gas mixture and conditions. Fromthe plots of selectivity for H₂ S removal shown in FIG. 5, it can beseen that MTBEE has a higher H₂ S selectivity than TBDEA.

While all examples herein illustrate the superior performance of theamino compounds for selective H₂ S removal using an absorber unit asrepresented by FIG. 2, it will also be possible to achieve effectiveselective H₂ S removal by using the amino compounds in anabsorption-regeneration unit as depicted in FIG. 1.

In summary, this invention is seen to provide a special class oftertiary amino compounds characterized as severely sterically hinderedtertiary amino compounds having a pK_(a) value at 20° greater than 8.6,preferably ranging from 9.3 to about 10.6, and a cumulative ⁻ E_(s)value of at least 1.9, and having a high selectivity for H₂ S inpreference to CO₂ which selectivity is maintained at high H₂ S and CO₂loading levels.

These amino compounds are capable of reducing the H₂ S in gaseousmixtures to a relatively low level, e.g., less than about 200 ppm andhave a relatively high capacity for H₂ S, e.g., greater than about 0.2mole of H₂ S per mole of amine. The amino compounds are characterized ashaving a "kinetic selectivity" for H₂ S, i.e., a faster reaction ratefor H₂ S than for CO₂ at absorption conditions. In addition they have ahigher capacity for H₂ S at equivalent kinetic selectivity for H₂ S overCO₂. This higher capacity results in the economic advantage of lowersteam requirements during regeneration.

Another means for determining whether a tertiary amino compound is"severely sterically hindered" is by measuring its ¹⁵ N nuclear magneticresonance (NMR) chemical shift. By such measurements it has been foundthat the "severely sterically hindered" amino compounds herein have a ¹⁵N NMR chemical shift greater than about δ+40 ppm, when a 90% by wt.amine solution in 10% by wt. D₂ O at 35° C. is measured by aspectrometer using liquid (neat) ammonia at 25° C. as a zero referencevalue. For example, N-methyl-N-tertiarybutylaminoethoxyethanol (MTBEE)has a ¹⁵ N NMR chemical shift value of δ°45.0 ppm, whereasmethyldiethanolamine (MDEA) has a ¹⁵ N NMR chemical shift value ofδ+27.4 ppm. As evident from the data shown herein, those amino compoundsmeasured having an ¹⁵ N NMR chemical shift value greater than δ+40 had ahigher H₂ S selectivity than MDEA having an ¹⁵ N NMR chemical shift lessthan δ+40 ppm.

The data in FIGS. 3 and 4 also show that the amino compounds of thepresent invention have very high capacity for both H₂ S and CO₂ comparedto methyldiethanolamine (MDEA) in addition to high H₂ S selectivities.It will be apparent from an inspection of the data in FIG. 3 that if theabsorption process is conducted under conditions such that the aminocompound has a long contact time with the gases to be absorbed, theselectivity for H₂ S decreases, but the overall capacity for both CO₂and H₂ S remains rather high. Therefore, one may, in some instances,wish to carry out a "non-selective" absorption process to take advantageof the large absorption capacity of the amino compounds of theinvention.

Accordingly, one may carry out a "non-selective" acid gas removalabsorption process using the amino compounds of the invention. Such"non-selective" processes are particularly useful in scrubbing naturalgases which contain relatively high levels of H₂ S and low to nil levelsof CO₂. As such, the amino compounds of the invention may replace someor all of the monoethanolamine (MEA) or diethanolamine (DEA) commonlyused for such scrubbing processes.

While the invention has been described in connection with specificembodiments thereof, it will be understood that it is capable of furthermodification, and this application is intended to cover any variations,uses, or adaptations of the invention following, in general, theprinciples of the invention and including such departures from thepresent disclosure as come within known or customary practice in the artto which the invention pertains and as may be applied to the essentialfeatures hereinbefore set forth, and as fall within the scope of theinvention.

What is claimed is:
 1. A severely sterically hindered tertiary aminocompound of the general formula: ##STR5## wherein R₁ and R₂ are eachindependently selected from the group consisting of alkyl radicalshaving 1 to 8 carbon atoms and hydroxyalkyl radicals having 2 to 8carbon atoms and cycloalkyl and hydroxycycloalkyl radicals having 3 to 8carbon atoms, R₃, R₄, R₅ and R₆ are each independently selected from thegroup consisting of hydrogen and C₁ -C₄ alkyl and hydroxyalkyl radicals,with the proviso that if the carbon atoms of R₁ and R₂ directly attachedto the nitrogen atom are primary, both R₃ and R₄ directly bonded to thecarbon which is bonded to the nitrogen are alkyl or hydroxyalkylradicals, and with the proviso that if the carbon atom of R₁ directlyattached to the nitrogen atom is secondary and the carbon atom of R₂directly attached to the nitrogen atom is primary, at least one of R₃ orR₄ directly bonded to the carbon which is bonded to the nitrogen is analkyl or hydroxyalkyl radical, x and y are each positive integersindependently ranging from 2 to 4, and z is either zero or a positiveinteger ranging from 1 to
 4. 2. A severely sterically hindered tertiaryamino compound of the general formula: ##STR6## wherein R is a tertiarybutyl, tertiary amyl or hydroxy tertiary butyl radical, x and y eachrange from 2-4 and z is 0 or
 1. 3. A severely sterically hinderedtertiary amino compound of the formula: ##STR7##
 4. A severelysterically hindered tertiary amino compound of the formula: ##STR8## 5.A severely sterically hindered tertiary amino compound of the formula:##STR9##
 6. A severely sterically hindered tertiary amino compound ofthe general formula: ##STR10## wherein R is a tertiary alkyl or hydroxytertiary alkyl radical having up to 8 carbon atoms, R₃, R₄, R₅ and R₆are each independently selected from the group consisting of hydrogenand C₁ -C₄ alkyl and hydroxyalkyl radicals, x and y are each positiveintegers independently ranging from 2 to 4, and z is either zero or apositive integer ranging from 1 to
 4. 7. The amino compound of claim 1having a pK_(a) value at 20° C. greater than 8.6 and a cumulative ⁻E_(s) value of at least 1.9.